LONDON -- An oil painting of artist Francis Bacon by his close friend Lucian Freud, one of only two of its kind, was sold on Sunday for more than $9 million, auction house Christie's said.

The unfinished and rarely-seen work, painted in 1956-7, fetched ?5,417,250 ($9,404,346 or ?6,972,001) at auction in London. It had been estimated at between five and seven million pounds.

It is the last known portrait of Bacon by Freud -- the other one, painted over three months in 1952, was stolen from an exhibition in Berlin in 1988.

The painting sold Sunday shows Bacon gazing downwards, as did the other portrait. Freud sat knee-to-knee with his subject as he painted, and the work offers an intimate glimpse into the friendship of the two great artists.

They first met in 1945 at the house of Graham Sutherland, a mutual friend and contemporary artist, and struck up a close friendship.

Bacon, who died in 1992, exercised great influence on the younger artist, and is credited with fuelling his desire to depict human life, Christie's said.

They sat for each other -- Bacon's first portrait of Freud was painted in 1951 and many others followed, although Freud painted his friend only twice.

According to the auction house, the first portrait was never finished because Bacon left the sitting suddenly, probably to go after his lover Peter Lacy in Tangiers, Morocco.

The earlier portrait was stolen from the Neue Nationalgalerie in Berlin in 1988, where it had been lent from Britain's Tate museum for a Freud retrospective, and has never been recovered.